1. Field of the Invention
The present invention relates to inductive data links, and particularly but not exclusively to data links and transmission protocols that are used in conjunction with implantable medical devices.
2. Related Art
Inductive links are used in many applications, but have particular application to implanted medical devices such as pacers, cochlear implants and other neural stimulators, and other types of devices. They allow for data and/or power to be transmitted to a device which is wholly implanted from an external system, without requiring a cable or connection to pass through the skin.
Hearing aids and cochlear implants are useful in restoring the sensation of hearing to hearing impaired individuals. This is an area where inductive links are widely applied.
A cochlear implant is used where the hair cells of the cochlea have been damaged to the extent that they are no longer able to convert the mechanical vibration of the cochlea fluid into an electrical signal. The cochlear implant generally bypasses the hair cells of the cochlea and delivers electrical stimulation, representative of speech and environmental sounds, to the nerves in the cochlear. The neural impulses generated by the electrical stimulation are then sent to the brain, thereby allowing the brain to perceive a hearing sensation.
The functional components of a cochlear implant system typically include an external sound processor device connected to an external headset/transmitter coil, and an implanted receiver-stimulator connected to an implanted electrode array.
In operation, the sound processor device detects sound and converts this into a binary coded, serial data stream, representative of the desired stimulation parameters. The serial data stream then modulates a high frequency (RF) analogue carrier. Typically, the serial data stream is 100% amplitude modulated on the analogue carrier, for example as is described in U.S. Pat. No. 4,532,930 (“Crosby”) or U.S. Pat. No. 5,741,314 (“Daly”). The contents of these documents are incorporated herein by reference.
The modulated signal is then output from a number of RF output driver circuits in the speech processing device, and sent to the headset/transmitter coil.
The headset/transmitter coil is frequency-tuned as a series resonance circuit that is inductively linked with a parallel-tuned resonant circuit in the form of the receiver coil in the receiver-stimulator. A voltage waveform thus induced in the receiver coil has a voltage waveform envelope representing the serial data steam.
A problem with such inductive communication links is that errors can occur when the modulation depth of the waveform received in the receiver coil falls below a certain threshold.
The modulation depth of the received waveform depends on a number of factors.
One factor is the tuned frequency and quality factor of the transmitter and receiver coils, since the higher the effective quality factor, the lower the modulation depth of the received waveform and the longer the after-burst ringing in both the transmitted and the received waveforms.
Another factor is the effective coupling coefficient between the transmitter and receiver coils. The modulation depth of the received waveform decreases with an increased distance between the two coils.
The prior art has employed a ‘tri-state’ function to reduce such data errors. Basically, this involves open-circuiting the output switches of the RF drivers during transmission of data “zeros”, inter-phase gaps and/or inter-frame gaps. Conversely, the output switches of the RF drivers are close-circuited during transmission of data “ones”. This ‘tri-state’ function reduces the effective quality factor of the transmitter coil, increases the resonance frequency, thus creating a higher frequency post RF-burst ringing that decays quickly. The interaction of the transmitted signal with the receiver coil leads to a larger modulation depth and thus improved data integrity.
However, a drawback of the prior art tri-state function is that an excessive voltage overshoot is generated when the output switches of the RF drivers are open-circuited. For example, RF switches operating with a supply voltage of 3.3V will produce an RF output voltage swinging between −0.8V and 4.1V, as a result of the output voltage overshoot when the switches are opened. The voltage of the RF drivers exceeds the power supply rails by a forward diode drop, typically about 0.8V above VDD and 0.8V below VSS.
This voltage overshoot can affect the reliability of the integrated circuit since the output switches may be overstressed from being operated outside the maximum operating voltage specified by the integrated circuit manufacturer. Further, this overstressing can gradually weaken, and eventually permanently damage the output switches.
One technique used to reduce the effects of this voltage overshoot is to provide protection diodes at each of the output switches. However, these diodes only share the current during the voltage overshoot and do not affect the voltage by any significant amount. Further, voltage waveform clamping and current conduction through the diodes defeats the purpose of the tri-state function until the signal drops below the clamping point.